1. Field of the Invention
The present invention relates to mineral exploration, and more particularly to methods of detecting deep massive sulphide deposits by the use of seismic reflection surveys.
2. Description of the Related Art
Multidimensional seismic reflection survey techniques, for example two dimensional (2-D) and three dimensional (3-D) seismic reflection survey techniques, have allowed the petroleum industry to generate remarkably accurate structural and stratigraphic subsurface geologic models. This can be achieved because multiple reflection events are recorded from the highly reflective sedimentary stratigraphy. As a consequence of this non acoustic transparency, the industry has been able to discover oil and gas deposits several thousand meters below the earth""s surface, making deep oil drilling an economically viable procedure.
The minerals industry has made a considerable investment in multi-dimensional seismic reflection techniques to adapt the procedure for detecting the presence of deep massive sulphide deposits. However, the use of multi-dimensional seismic technologies by the minerals industry has thus far been met with comparatively less success. In most cases, a massive sulphide is extremely difficult to discern within the complex acoustic wavefield generated by the other stratigraphic contracts of the geological region being explored.
To date, mineral exploration using seismic reflection surveys has been typically done on a broad spectrum of geological terrain known to be potential host sites for massive sulphide deposits Unfortunately, many of these regions exhibit poor signal to noise conditions. To date, this relatively poor data has been found to be an inherent feature of multi-dimensional seismic exploration for massive sulphides. Experts in die field have worked diligently to overcome these problems by using state-of-the-art acquisition and processing strategies derived from the petroleum industry, hoping to filter out the noise and to generate an accurate and reliable subsurface geologic model based on the remaining relatively poorly understood acoustic signature of the massive sulphide deposit. With this goal in mind, experts have developed a number of sophisticated procedures which have been published in a number of leading mining journal articles, many of which are listed herein below.
However, these investigations have thus far not yielded significant useful results.
It is an object of the present invention to provide a novel technique for prospecting for massive sulphides.
Briefly stated, the invention involves a technique of prospecting for deep massive sulphide ore bodies, comprising the steps of:
selecting a geologic region which is substantially acoustically transparent;
directing seismic waves at the region and collecting reflected and diffracted waves therefrom; and
analyzing the collected waves for the presence of the massive sulphide ore bodies.
In one presently preferred aspect, the present invention provides an exploration process for evaluating a plurality of geological regions that are candidates for massive sulphide deposits. The process includes the steps of obtaining impedance characteristics for corresponding strata located in a selected one of said geological regions; analyzing the impedance characteristics for determining the presence of impedance transitions from one of the strata to another; validating the presence of a host lithology in said strata of the selected region based on said impedance transitions by determining whether said host lithology is substantially acoustically transparent, such that the host lithology is represented by a seismic profile which contains a substantial absence of impedance markers of magnitude greater than that of any survey noise present in said impedance characteristics; and confirming the selected region possesses a host stratigraphy that is substantially acoustically transparent and is therefore a candidate for a subsequent seismic investigation for massive sulphide deposits.
In another of its aspects, the invention provides a method of exploration for evaluating a plurality of geological regions for the suitability of direct detection of a massive sulphide deposit. This method includes the steps of obtaining impedance characteristics for corresponding strata located in a selected one of said geological regions; analyzing the impedance characteristics for determining the presence of impedance transitions from one of the strata of the selected region to another; validating the presence of a host lithology in said strata based on said impedance transitions by determining whether said host lithology is substantially acoustically transparent, such that said impedance transitions from said strata primarily contain only survey noise; and confirming the selected region possesses a host stratigraphy that is substantially acoustically transparent and is therefore a candidate for a subsequent seismic exploration for detection of the massive sulphide deposit.
Rather than simply applying the latest multi-dimensional seismic techniques on all geologic regions suspected of bearing massive sulphide deposits, the present technique involves selecting only those geologic regions which have a particular range of characteristics, primarily those suspected of having a local host stratigraphy which is essentially acoustically transparent, and then applying seismic survey techniques only on those selected geologic regions.
The term xe2x80x98acoustically transparentxe2x80x99 as a characteristic of a local host stratigraphy, refers to the ability for seismic waves to pass through the statigraphy while producing minimal or otherwise substantially non-interfering seismic reflections from geological boundaries which are not the boundaries of massive sulphide contacts. For example, an acoustically transparent local host stratigraphy may be one in which a massive sulphide deposit generates a recognizable peak (positive amplitude) on stacked seismic refection trace that exceeds the amplitude of noise from the survey, for example the noise originating from the local host rock itself, that is geologic noise, and the ambient noise originating from the seismic survey source and environment, The local host stratigraphy, in this case, is that which is in the vicinity of he massive sulphide itself, That is above and below the massive sulphide and not necessarily the entire depth investigation range of the geological terrain. In this case, a major xe2x80x98markerxe2x80x99 may be above or below the local host stratigraphy and could present a significant peak in an impedance trace but not otherwise substantially interfere with The acoustic transparency of the adjacent local host stratigraphy itself. This geologic marker could be used to help focus the search for anomalous seismic amplitudes to that zone of the stratigraphic column most likely to host deep massive sulphide deposits. For example, in some cases, a local host statigraphy may be essentially acoustically transparent if the seismic amplitude anomaly, resulting from a sulphide orebody, is approximately twice that of contacts within the local host stratigraphy.